docs: def warn(filepath, expected):
          """Raise warning.
  
          Parameters
          ----------
          filepath : path-like
              Given filepath.
          expected : string
              Expected file suffix.
          """
          filepath = pathlib.Path(filepath)
          message = "file {0} has type {1} (expected {2})".format(
              filepath, filepath.suffix, expected
          )
          warnings.warn(message, WrongFileTypeWarning)